This invention relates to a printed antenna for microwave transmission and reception.
The printed antenna in which antenna elements and a transmission line are formed on a printed board has many advantages that it can be thin, light and small, it can be made in mass-production, and it can be formed integrally with electronic circuits. Such a printed antenna is used as an antenna for microwave transmission and reception of satellite broadcast, movable body communication or the like. There are various types of printed antennas. It has been increasingly noted that one type of printed antenna, in which a linear strip antenna element is used and a window is provided in a grounded conductor by cutting off the same to obtain a broad or wide band, is stable for its operation since it has only one resonance mode, compared with another type of printed antenna utilizing a patch type of element.
Furthermore, in case where a linear strip antenna element is used as an antenna for circularly polarized wave transmission and reception for use a satellite broadcasting transmission and reception, it has been proposed that the strip antenna element be combined with a slot antenna of a linear element similar to the strip antenna element, and the slot antenna element is positioned relative to the strip antenna element so that a power supplying phase difference between the strip and slot antenna elements is made 90.degree. along the transmission line for excitation. In such a case, since the electric field radiated from the strip element and electric field radiated from the slot element are spatially perpendicular to each other, these electric fields have a phase difference of 90.degree. in time, and at the same time, constitutes a combination of spatially crossed oscillating electromagnetic field to effectively radiate a circularly polarized wave. Although the explanation of the antenna is directed to a transmitting antenna, it should be understood that the transmitting antenna can also be used as a receiving antenna due to duality of electromagnetic field.
Such a printed antenna constructed by a combination of linear elements has a feature that it is stable for its operation as mentioned above, and in addition to that feature, it can electronically switch over between waves of right and left circular polarization, between a polarized vertical wave and a polarized horizontal wave, or between a circularly polarized wave and a linearly polarized wave for use in a satellite broadcast utilizing a satellite communication. Consequently, it has a feature that it can perform multi-functions compared with another printed antenna using a patch type of elements to be designed for transmitting and receiving a circularly polarized wave for each element.
As mentioned above, the window formed by cutting off portions of the grounded conductor can widen or expand the frequency band of the strip antenna element. At the same time, an electromagnetic wave is radiated on both sides of the antenna from the window. In order to radiate an electromagnetic wave only on one side of the antenna, a reflector plate 20 is provided as shown in FIG. 20. The reflector plate may be provided on either side of a base plate or substrate. However, in order to accomplish the purpose of reducing radiation loss from the transmission line, it is preferred that the reflector plate is provided on the side where the transmission line is positioned.
Furthermore, in order to prevent the deterioration of circularly polarized wave due to the provision of the reflector plate, the provision of a strip antenna in the window was proposed by the same inventor (Japanese Patent Application No. 344229/1989).
FIGS. 18 through 20 show a conventional printed antenna constructed by a combination of above-mentioned linear elements for radiating circularly polarized wave. Referring to these FIGS. 18 through 20, each linear strip element 10 is provided in the window 14 to effectively radiate an electromagnetic wave of frequency determined by length of linear strip antenna element by electromagnetic coupling between each linear strip element 10 and the transmission line 12. Since the window 14 is wide in its width and functions as a slot of a long length, spurious radiation is generated due to radiation from the strip antenna element 10. However, the spurious radiation can be suppressed by the provision of a pair of elements cancellation, not shown.
However, with the conventional antenna, complete cancellation of spurious radiation by use of the above-mentioned method can be made only in a certain direction and at a certain frequency. Furthermore, since gain decreases due to disturbance of radiation pattern generated by spurious radiation, a design for amending disturbance of radiation pattern at the whole arrayed antenna is required, and therefore there is a disadvantage that it is hard to design an antenna.